Nuts & Volts Magazine (November 2011)

Thoughtful Repurposing

Teardowns of electronics devices can be an excellent means of learning how to design circuits for the real world, as well as an inexpensive source of parts for your construction projects. In these times of economic constraints, it’s tempting to simply go for the parts and bypass the time-consuming circuit analysis. Armed with an old toaster oven, it’s relatively easy to heat circuit boards and knock off dozens of components with a tap.

However, if you invest just a few minutes trying to understand the circuit on each board, you’ll likely learn a few construction tricks, and save both time and money in the long term. Pay attention to the circuit layout on your next teardown. Are components bolted, glued, or simply friction fit? What’s the spacing? Are fuses on both the input and output? How are cables run and fished? More importantly, are there sub-circuits that can be repurposed in situ? That is, let’s say you’re preparing to harvest the components from a shortwave receiver. Why simply remove the transistor or IC, electrolytic caps, and resistors, only to have to recreate an audio amp circuit later? Better to keep the circuit intact, perhaps by cutting away and saving that section of the printed circuit board.

If harvesting part of the circuit board isn’t practical, then at least consider extracting the components of the audio amp and storing them separately, together with a schematic of the original circuit. Then, when it’s time to add an audio amp to your next project, you have a proven design and components to work with. I’ve found that power supplies tend to be easily reclaimed intact, and they’re also the most likely sub-circuit to be useful in future projects.

Another way to save on the cost of experimentation is to be on the lookout for boards and circuits that use leaded components. SMT components are nice and compact, but they’re very difficult to repurpose without resoldering. Leaded components, on the other hand, lend themselves to solderless breadboard circuit designs. Even if you reclaim sub-circuits, it’s often a simple matter to add either short lengths of solid wire or stiff 0.1” headers to make the section of board ‘plug and play’ with solderless breadboards.

Back to the topic of component removal. I’ve found an old toaster oven is the fastest, most economical means of harvesting components from a circuit board. I say old because the oven is dedicated to heating solder to the melting point. The solder fumes — together with the gassing of components — leaves unhealthy residues inside the oven. No point in exposing yourself or your family to extra doses of heavy metals.

The trick with using a toaster oven is knocking the components out of the board as soon as possible. I use a silicon baking sheet to catch hot components, but you can use an old pizza pan. Consider grounding the pan if you’re working with sensitive components. Toaster ovens vary in temperature, but I’ve had luck with the oven set to 450°. Solder begins to flow in about three minutes.

Barring use of an oven, a hot air pen is very effective, but time consuming. It’s my tool of choice for delicate, multi-legged ICs. That said, I’ve harvested components from dozens of boards using an old-fashioned pencil soldering iron pen. Just keep a used tip handy to use for desoldering — no need to sacrifice a new tip for old parts. NV